If you really want to use 128GB of RAM, and I mean - really go that high on computations - then just putting in 128GB of DDR4 3000 is not gonna cut it IMO. The transfer rate and the CPU's computational capacity will become the main bottleneck, not the amount of RAM.
If you really wanna go that far, then might do something like this :
PCPartPicker part list / Price breakdown by merchant
This is Dual-Xeon rig. E5-2620 has stock clocks of 2.4, but will turbo up to 3.2 if properly cooled and powered. You have 2 of them, which gives you 12 cores, and 24 threads. This also gives you quad-channel 128GB DDR4 2133 RAM, with peak bandwith of 17GB/s per channel x 4 = 68GB/s. Also, the RAM is ECC (error correcting) which may or may not be important to you.
Its 400$ bump over your budget, but the performance improvements are quite "dramatic".
Other than that - I believe you should be fine with 64GB. But its up to you, really.
By the way - sorry, I misred your request, and offered the 32GB build. If you want 64GB, the build with i7-5820k should suit you well.